Method of improving mechanical properties of semiconductor interconnects with nanoparticles

ABSTRACT

In a BEOL process, UV radiation is used in a curing process of ultra low-k (ULK) dielectrics. This radiation penetrates through the ULK material and reaches the cap film underneath it. The interaction between the UV light and the film leads to a change the properties of the cap film. Of particular concern is the change in the stress state of the cap from compressive to tensile stress. This leads to a weaker dielectric-cap interface and mechanical failure of the ULK film. A layer of nanoparticles is inserted between the cap and the ULK film. The nanoparticles absorb the UV light before it can damage the cap film, thus maintaining the mechanical integrity of the ULK dielectric.

FIELD OF THE INVENTION

The present invention relates generally to semiconductor integratedcircuits and devices, and more particularly, to the application ofnanoparticles in semiconductor interconnect processing.

BACKGROUND

Semiconductor based devices and circuits consist of active devices,typically transistors, on a silicon wafer surface and a set ofconducting wires interconnecting them. This set of wires is typicallyreferred to as the back-end-of-line (BEOL) while the active transistorsare referred to the front-end-of-line (FEOL). A complex network ofconducting interconnects is desired in order to electrically wire up thelarge number of devices, thus creating functional circuits. This isaccomplished by building a multi-level structure consisting of metalliclines embedded in an insulating dielectric medium. Modern high speedinterconnects typically consist of copper (Cu) conductors which areinsulated from one another by low dielectric constant (low k) materials.The interconnect structure may consist of as many as fifteen verticallystacked levels of metal with conducting path between levels, calledvias. The wires are characterized by their line width and the distanceto the nearest neighbor. The sum of this wire width and space isreferred to as the pitch. The first few levels of wiring are built atthe minimum allowed technology pitch, as determined by lithography. Thetight pitch allows for building the densest circuitry, with the higherlevels built at multiples of the minimum pitch. This hierarchicalstructure allows for thick wide lines, also referred to as fat wires, atthe higher levels which are typically used for distributing signals andpower across the chip. In addition to serving as an electrical insulatorthe dielectric material provides mechanical support for the multilevelstructure.

At present, Cu/low-k multi-level structures are typically formed by dualdamascene processing as follows: the dielectric material is deposited asa blanket film, lithographically patterned, and then reactive ion etched(RIE), creating both trenches and vias. The pattern is then coated by arefractory metal barrier such as Ta and TaNx followed by a thinsputtered copper seed layer. The seed layer allows for theelectrochemical deposition (ECD) of a thick copper layer which fills upthe holes. Excessive copper is removed and the surface is planarized bychemical mechanical polishing (CMP). Lastly, a thin dielectric film alsoknown as ‘cap’ is deposited over the patterned copper lines. This dualdamascene process is repeated at each of the higher levels built.

As predicted by Moore's law, semiconductor devices continue to scaledown in order to improve device performance and place more transistorson the substrate. The corresponding scaling of the interconnectstructure causes an increase the parasitic resistance (R) andcapacitance (C) associated with the copper/low-k interconnects. The RCproduct is a measure of the time delay introduced into the circuitry bythe BEOL. In order to reduce the RC delay, low-k and ultra low-k (ULK)materials are used as dielectrics.

A typical type of low-k dielectric is an organo-silicate glass material,also referred to as SiCOH. It consists of cross-linked SiO₂-liketetrahedral structures as the backbone and some —CH₃ or —H as theterminal groups or side chains to lower polarizability, introduceporosity and reduce volume density. The low-k dielectrics are typicallydeposited by plasma enhanced chemical vapor deposition (PECVD) process,which mixes the organic precursor for sacrificial porogen (e.g.,cyclohexene, and the like) and the matrix precursor for the low-kbackbone structure. (e.g., decamethylcyclopentasiloxane,diethoxymethylsilane, dimethyldimethoxysilane,tetramethylcyclotetrasilane, octamethylcyclotetrasilane, and the like).The deposition step is followed by an ultraviolet (UV) curing process toremove the volatile organic porogen which is loosely bonded to the low-kbackbone. As a result, porosity is introduced into the low-kdielectrics. In addition, the UV curing process also induces thecross-linking of low-k dielectrics, improving the mechanical strength.However, ULK films are known to be mechanically weaker than theirnon-porous low-k counterparts. With porosity and reduced dielectricconstant comes a reduction in the film's Young modulus. Typical ULKmoduli are in the 2-8 GPa range, depending on the degree of porosity,making the ULK films especially susceptible to mechanical stressesduring BEOL processing and during chip packaging.

The dielectric film, which caps the top of the damascene metalstructure, prevents copper out-diffusion into the surrounding low-kdielectric. From the perspective of performance and reliability,physical and electrical properties of the cap dielectric, such asbreakdown voltage, adhesion to underlying metal and dielectrics,hermeticity, internal stress and elastic modulus, are very important. Ingeneral, mechanically compressive films with good adhesion to copperhelp suppress Cu electromigration and provide a mechanically robuststructure. Denser compressive films also tend to have a higher breakdownvoltage and provide enhanced hermeticity and passivation of the copperlines. A typical dielectric barrier used in advanced semiconductormanufacturing is an amorphous nitrided silicon carbide (SiCNH).

The UV radiation used in the curing process of ULK dielectrics ranges inwavelength from 200 nm to 600 nm and is generated by a UV bulb,illustrated hereinafter with reference to FIG. 1. The radiation canpenetrate through the ULK film and damages the SiCNH cap leading to achange in its mechanical stress state from compressive to tensile. Thisin turn can lead to spontaneous cracking of the porous ULK materialabove the cap and to poor reliability during chip packaging operation.An existing solution is to replace a conventional single layer SiCHNwith a bilayer low-k cap. This solution has two issues: first, thestress state of the cap still changes albeit at a slower rate. The filmstays compressive only if the UV cure time is short (<70 sec). TypicalULK cure times are greater than 100 sec. For these longer cure times,the bilayer cap stress state turns tensile; and second, the bilayer capwith a nitrogen-rich SiCNH on the bottom and carbon-rich SiCNH at thetop tends to shrink under UV radiation. A rough estimate is about 2%thickness under 70 s of UV cure while the thickness change of high-k(standard) SiCNH is around zero under the same irradiation conditions.This shrinking of the cap is undesirable and can lead to additionalmechanical stresses on the BEOL structure.

Referring to FIG. 2, the internal stress change (measured in MPa) of aSiCHN cap film due to exposure to UV during the ULK cure is illustrated.The stress measurement is shown for different cap materials at differentUV cure times. More specifically, the stress change due to the exposureto UV during the ULK curing process shows that the internal stresschanges from a negative value (compressive stress) to a positive value(tensile stress) as the UV cure time increases. The curve identified asSiCNH high-k represents a conventional deposition process, while thesecond curve referenced as SiCNH low-k represents the bilayer depositionprocess. Although the bilayer cap can slow down the stress change rate,the film ultimately turns tensile. (i.e. crosses the y-axis fromnegative to positive values) This change from compressive to tensilestress can be understood in terms of a bond breaking mechanism in theSiCHN film upon absorption of the high energy UV photons. The resultingbroken, also known as dangling, bonds lead to an increase in internalopen spaces and a reduced compressive stress. Tensile films are moreprone to cracking and loss of adhesion to the underlayer pattern.

FIG. 3 shows the thickness of the bilayer low-k cap shrinked uponexposure to UV radiation. This cap film loses about 2% of its initialthickness due to the loss of bonded hydrogen and carbon groups in thefilm. Conventional high-k SiCNH does not shrink in thickness upon asimilar exposure to UV radiation.

Referring to FIG. 4, the UV-VIS absorption characteristics of typicalmetal oxide, e.g., ZnO dispersed in double distilled water along-withthe associated reactants are shown. Curves 1, 2, and 3 correspond to PVP(polyvinylpyrrollidone used to prevent agglomeration), ZnOnanoparticles, and Zn(NO₃)₂, respectively. ZnO nanoparticles can besynthesized by way of different types of alcoholic solutions such asmethanol, ethanol, propanol, or higher alcohols.

X-ray diffraction, TEM, and EDAX are used to verify the formation of ZnOnanoparticles. The absorption peak for the ZnO nanoparticles is observedat 262 nm, which lies below the bandgap wavelength (shown as the dottedline in FIG. 4) of 388 nm (E_(g)=3.2 eV) of bulk ZnO. The shift in theabsorption edge to lower wavelength is a fundamental property ofnanoparticles and is attributed to a widening of the bandgap whenparticle sizes become small.

Referring to FIG. 5, the nanoparticle size effect on peak absorbancewavelength is illustrated for various particle diameters. The ZnOnanoparticles show peak absorbance at approximately 262 nm when theaverage particle size is 2.1 nm. The circular point in FIG. 5 indicatesthe value of the average particle size as obtained from TEM analysis.From the foregoing, it is evident that ZnO nanoparticles exhibitsignificant confinement effects for particle diameters less than about 8nm.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and whichconstitute part of the specification, illustrate the presently preferredembodiments of the invention which, together with the generaldescription given above and the detailed description of the preferredembodiments given below serve to explain the principles of the, whereinlike reference numerals denote like elements and parts, in which:

FIG. 1 is a plot of radiant power vs. wavelength showing the spectraloutput of a UV bulb, as it is known in the prior art;

FIG. 2 is a plot showing the internal stress change of a single layerand a bilayer SiCHN cap film due to exposure to UV radiation during ULKcure, as it is known in the prior art;

FIG. 3 shows a graph, plotting the percent film thickness shrinkage of abilayer low-k cap vs. UV cure time. This shrinkage is attributed to theloss of bonded hydrogen and carbon, as it is known in the prior art;

FIG. 4 shows UV-VIS absorption characteristics of ZnO nanoparticles, asit is known in the prior art;

FIG. 5 illustrates the effect of the size of nanoparticles (e.g., ZnOnanocolloid) on peak absorbance wavelength, as known in the prior art;

FIG. 6 illustrates a side cross sectional view of a structure showing amulti-distributed size nanoparticles between an ultra low-k (ULK)dielectric and a SiCNH cap, allowing UV rays with broad wavelength to beabsorbed, according to one embodiment of the present invention;

FIG. 7 shows a side cross sectional view of converting the structureinto a multi-level interconnect configuration by deposition of a secondULK level with UV cure;

FIG. 8 is a side cross sectional view of one embodiment showing theformation of a TEOS HM layer for lithographic purposes, followed by anULK etch to form trenches and vias followed by a selective removal ofthe nanoparticles;

FIG. 9 shows a side cross sectional view illustrating an in-situ Cu seedand TaN/Ta liner deposition, followed by electroplated copperdeposition, filling the trenches and vias;

FIG. 10 is a side cross sectional view showing a final BEOL stackdepicting the multi-level structure nanoparticles placed in theinterface between the ULK dielectric and SiCNH cap, in accordance withan embodiment of the invention.

SUMMARY

In one aspect, the invention described the insertion of nanoparticlesbetween the cap material covering the metal interconnects and the ULKinsulating dielectric material above it. The use of nanoparticles leadsto reduced ultraviolet (UV) radiation damage during processing. In theabsence of radiation damage, the cap material remains mechanicallycompressive thus increasing the strength of the BEOL structure.Ultraviolet radiation used in an embodiment of the present invention ispreferably generated by a UV bulb having a spectrum ranging from 200 nmto 600 nm with a large proportion of the wavelength preferably below 400nm.

A layer of multi-sized nanoparticles is inserted between the cap and ULKto absorb UV radiation during the ULK curing process. The nanoparticlesare preferably made of a metal oxide with a diameter ranging from 1 nmto 4 nm. This range of nanoparticle sizes ensures a high absorption ofUV radiation up to approximately 375 nm. The absorption edge for SiCNHfilms as a function of the carbon content of the films is approximately400 nm for 26% carbon. In one embodiment, the films range between 20-26%carbon as determined by RBS. Radiation with wavelength longer than 400nm is transmitted through the SiCNH films and cannot cause the damagewhich drives the films into tensile stress. On the other hand, about 90%of the UV bulb spectrum that lies between 200 nm to 400 nm that candamage the SiCHN films is absorbed by the nanoparticles.

The formation of a monolayer of nanoparticles is instrumental inproviding a robust structure capable of handling mechanical stresses,particularly during the manufacture of semiconductor back end (BEOL) andsubsequent integration. A monolayer of nanoparticles is provided,preferably made of metal oxide, such as ZnO or TiO₂, and capable ofattenuating and absorbing UV radiation used in the formation of ultralow-k dielectrics.

In a further aspect, in one embodiment of the present invention, thenanoparticles are placed between a SiCNH cap and an ultra low-kdielectric such that UV radiation is absorbed, the cap being protectedfrom UV damage and the associated compressive to tensile stress change.(Note: the cap applies to the next level, serving as a base for the nextnanoparticle deposition, since there is a need for nanoparticlesprotection against UV at every ULK level of the multilevel structurebeing built). The size of the nanoparticles is adjusted to absorb UVradiation efficiently at certain wavelengths.

In still another aspect, the invention provides nanoparticles spun onthe cap, and dried by removing the solution, the solution being made ofmethanol or other organic alcohols.

In yet another aspect, one embodiment of the invention includes: a)spinning nanoparticles and drying them out by way of an organicsolution, the solution made of methanol or other alcohols; b) depositingthe next level ULK with a UV cure; c) ULK etching followed by removingnanoparticles with organic solvents at open area created by the etching,using DHF to clean solvents and other residuals; d) in-situ depositingTaN/Ta liner and Cu seed followed by an electroplated copper and ananneal; and e) removing the overburden (excess) Cu/liner by CMP,followed by depositing a SiCNH cap layer.

In still a further aspect, the invention replaces the UV inflicteddamage to the cap that becomes tensile under UV radiation from higherlevels, which leads to the formation of cracks at the ULK/cap interface.Introducing a bilayer low-k cap operates only within certain UVconditions, the thickness thereof shrinking under the UV radiation.Furthermore, dimensional changes may undermine the integrity of the BEOLstructure.

In another aspect, an embodiment of the invention provides a multi-layerstructure that includes: one or more ultra low-k (ULK) dielectric layerswith each alternating ULK dielectric layer having a plurality of metalfilled trenches and vias formed therein; a cap, capping and sealing theULK dielectric layers having the plurality of metal filled trenches andvias; and nanoparticles forming a monolayer at an interface between theULK dielectric layers and the caps.

In still another aspect, the invention provides a method of forming amulti-layer structure that includes: forming one or more ultra low-k(ULK) dielectric layers with each alternating ULK dielectric layerhaving a plurality of metal filled trenches and vias formed therein,forming a cap, capping and sealing the ULK dielectric layers having theplurality of metal filled trenches and vias; and spin cappingnanoparticles forming a monolayer at an interface between the ULKdielectric layers and each of the caps.

DETAILED DESCRIPTION

The present invention will now be described in greater detail by way ofthe following discussion with reference to the drawings that accompanythe present application. It is observed that the drawings of the presentapplication are provided for illustrative purposes only.

One embodiment of the present invention will be described hereinafter.For simplicity and clarity of illustration, elements shown in thedrawings have not necessarily been drawn to scale. For example, thedimensions of some of the elements may be exaggerated relative to otherelements for clarity.

FIG. 6 illustrates an embodiment of the inventive structure showingnanoparticles 120 spun on top of the SiCNH cap layer 130 on top of Culayer 140, wherein the nanoparticles are preferably dried out by organicsolutions. The SiCNH film can be made with a thickness ranging between150 A and 500 A, and is used to cap the damascene copper pattern below.The ULK layer 100 spun over the nanoparticles is preferably made of alow K organo silicate dielectric such as SiCOH, with a thickness rangeof 500 A-10000 A.

The nanoparticles 120 are preferably made of metal oxides, e.g., ZnO. Itis understood that other materials with similar characteristics, such asTiO₂ and the like, may be advantageously used. The preferred diameter ofthe nanoparticles ranges between 1 nm and 4 nm. The nanoparticles areuniformly spun (spin-coated) forming a monolayer. It has been shown thatZnO or TiO₂ nanoparticles can be successfully synthesized in differenttypes of alcoholic solutions, including methanol, ethanol and higheralcohols.

The size of the nanoparticles can be adjusted to absorb efficiently theUV radiation at certain wavelength frequency. As previously described,the purpose of the nanoparticles is to protect the SiCNH cap from UVrays radiation, therefore protecting the cap from changing its internalstress from compressive to tensile. The as-deposited cap is compressive.Upon exposure to UV radiation the cap turns tensile as previously shownin FIG. 2. Tensile films tend to crack more easily than compressivefilms. A crack in the cap film can induce cracking in the low modulusULK film with which it is in contact thereof. The nanoparticles arepreferably made having multi-distributed sizes (mostly 1 nm to 4 nm) sothat most of the UV can be absorbed efficiently.

Peak absorption, as was previously shown with reference to FIG. 5 varieswith particle sizes. The absorption peak shifts to shorter wavelength asthe particle size decreases due to a quantum confinement effect. Theintent is to absorb radiation with wavelength shorter than about 400 nm,wherein the UV bulb, as previously shown with reference to FIG. 1, putsout most of its UV radiation, and where the SiCHN cap has an absorptionedge.

Radiation is absorbed by the nanoparticles through electronictransitions from the top of the valence band up to the conduction band.The excited electrons drop back to the valence band, typically by asequence of transitions through defect states in the optical band gap.The resulting photo-luminescence spectra peaks around 550 nm (visiblelight) for ZnO.

Referring to FIG. 7, trenches having a critical dimension (CD) ranging40 nm to 1000 nm are then formed at selected locations, having the sidesthereof lined by way of a liner 170, preferably, made of but not limitedto Ta/TaN, and having a thickness of approximately 10 nm or less. Then,the trenches are coated preferably with PVD (physical vapor deposition)Cu seeds, using for instance sputtering, and having a thickness of theorder of about 100 nm or less. This is followed by electroplated copperfill 180.

The structure thus formed is converted into a multi-level interconnectconfiguration. A new ultra-low-k layer 100′ similar to the previous one,is deposited preferably using PECVD on top of monolayer of nanoparticles120 (i.e., with the sizes ranging 1 nm-4 nm) followed by UV cure Aspreviously described, the upper ULK level may once again be altered byadding trenches and vias to provide additional interconnects or byconnected the Cu connect to previous level(s).

The foregoing is illustrated with reference to FIG. 8 that illustratesthe upper level by ULK etch, removing the nanoparticles preferably withorganic solvents at the open areas created by RIE etching, followed byDHF (dilute hydrofluoric acid) to clean the solvents and otherresiduals. The process is advantageously achieved by providing the toplevel with a deposition of a TEOS hardmask (HM) 160, for lithographicpurposes, preferably formed using PECVD. The thickness of the TEOS HMmay preferably range from 15 nm to 50 nm. Depicted in FIG. 8 are alsoshown several trenches 150 and vias 155 formed by etching, including aselected removal of the nanoparticles, where there is a need to makecontact with the Cu plated trenches in the first ULK substrate. Thepurpose of the additional trenches thus formed is to make it possible toform a multilevel network of interconnects necessary for VLSI chips.

Referring to FIG. 9, the previously described TEOS HM deposition 160 isshown having an in-situ TaN/Ta liner deposition 185 followed by Cu seeddeposition and ECP copper 190, filling the trenches and vias and ananneal. The structure thus formed is preferably planarized using CMP.The electro-chemical plating is used to fill the dual damascene trencheswith copper, is typically carried out at room temperature (25° C.). Thecopper seed layer is deposited using physical deposition techniques,such as sputtering, consisting of copper alloys (i.e. Aluminum,Manganese or other alloying elements).

Referring to FIG. 10, there is shown a scheme of the final BEOL stackwith nanoparticles between the low-k and SiCNH cap. The structure ispolished and planarized by way of CMP. The process is followed by theSiCNH cap deposition. Additional monolayers of nanoparticles can beformed on top of each subsequent combination of ULK layer topped by aSiCNH cap, thereby creating a multi-level structure.

To summarize, the present description describes a multilevel structureto create a BEOL stack, typically of the order of 15 to 22 levels. Apreferred process used for each level forming the stack, includes:

1. Spinning nanoparticles and drying out the solution;

2. Depositing the next level ULK followed by UV cure;

3. Lithographic patterning of lines or vias;

4. Transferring of line or via pattern into the ULK dielectric byetching;

5. Removing the nanoparticles at the bottom of the open areas withorganic solvents, (i.e., open areas formed by the RIE process, whereinlines and vias are opened in the ULK film);

6. Diluting an HF (DHF) rinse of solvents and RIE residuals;

7. In-situ depositing a TaN/Ta liner and Cu seed by physical vapordeposition (PVD), followed by an electrochemical plating (ECP) of Cu,and annealing said Cu;

8. CMP the copper and liner overburden;

9. Depositing a SiCNH cap; and

10. Returning to step 1.

While the present invention has been particularly shown and describedwith respect to preferred embodiments thereof, it will be understood bythose skilled in the art that the foregoing and other changes in formsand details may be made without departing from the spirit and scope ofthe present invention. It is therefore intended that the presentinvention not be limited to the exact forms and details described andillustrated, but fall within the scope of the appended claims.

The invention claimed is:
 1. A method of forming a multi-layer structurecomprising: forming one or more ultra low-k (ULK) dielectric layers witheach alternating ULK dielectric layer having a plurality of metal filledtrenches and vias formed therein, forming a cap, capping and sealingsaid ULK dielectric layers having said plurality of metal filledtrenches and vias; and spin capping nanoparticles forming a monolayer atan interface between said ULK dielectric layers and each of said caps.2. The method as recited in claim 1 further comprising: a. spinning-on asolution on said nanoparticles, followed by drying out saidnanoparticles of solvent; b. depositing a next level ULK with UV cure;c. etching said ULK and removing said nanoparticles from etched openareas; d. depositing a refractory liner and a conductive layer followedby electroplated copper filling said open areas, followed by annealing;and e. polishing said liner and conductive layer; and f. depositing adielectric cap thereon.
 3. The method as recited in claim 2, whereinsaid conductive layer is made of Cu or Cu alloy.
 4. The method asrecited in claim 2, wherein said dielectric cap is made of SiCHN.
 5. Themethod as recited in claim 2, wherein said drying is carried out byheating.
 6. The method as recited in claim 2, wherein said drying out ofsaid nanoparticles is preformed using an organic solvent.
 7. The methodas recited in claim 2, wherein said polishing is followed by cleaningsaid solvents and other residuals.
 8. The method as recited in claim 2,wherein said refractory liner is made of TaN, Ta, W, WNx, TiNx, Ru, orCo.
 9. The method as recited in claim 7, wherein cleaning said solventsand residuals is performed using dilute hydrofluoric acid (DHF).
 10. Themethod as recited in claim 1, wherein said trench filling is performedby electrochemical plating.
 11. The method as recited in claim 2,wherein said polishing is performed by a chemical mechanical polish(CMP).
 12. The method as recited in claim 2 further comprising curingsaid ultra low-k (ULK) dielectrics.
 13. The method as recited in claim2, wherein radiation penetrates through said ULK dielectric, reachingsaid cap.
 14. A method of forming a multilevel Back-end-of-line (BEOL)stack, each level comprising: spinning a solution containingnanoparticles followed by drying out said solution; forming a ULKdielectric layer followed by UV cure; lithographic patterning lines orvias; transferring said patterned lines or vias into said ULK dielectricby etching; removing said nanoparticles at a bottom of open areascreated by rinsing with organic solvents; surface cleaning by applyingDHF and rinsing said solvents and said etching residuals; forming aliner made of TaN, Ta, W, WNx, TiNx, Ru, Co followed by Cu or Cu alloydeposition followed by electrochemical process of said Cu followed byannealing said Cu; and applying a chemical mechanical polish of saidcopper and liner overburden; and depositing a SiCNH cap.